There are several problems related to the use of enzymes and other bioreactants for therapy. These problems include immunogenicity, rapid removal, site of action, and the availibility of enzyme and bioreactants. With the advance of biotechnology, increasing number of enzymes and bioreactants can now be made available. There is therefore increasing interests to solve the other potential problems.
A number of centers have used immobilization of bioreactants to solve some of the problems mentioned (Chang, T. M. S. (ed) "Biomedical Applications of Immobilized Enzymes and Proteins" Vol. I & II, Plenum Publishing Co., New York, U.S.A., (1977); Chang, T. M. S. "Methods in the medical applications of immobilized proteins, enzymes and cells", Methods in Enzymology 137:444-457, (1988); Mosbach, K. (ed) "Immobilized Enzymes and Cells" Vol. 137 of "Methods in Enzymology", (1988); Widder, K. J. and Green, R. (ed) "Drug and Enzyme Targeting" Vol. 112 of "Methods in Enzymology" (1985)). Most of these immobilized bioreactants are used for extracorporeal blood or plasma perfusion. They are also used for parental injection especially intravenous injection. However, approaches using extracorporeal blood circulation or parental injection have their own problems. In parental injection one of the problems is related to introduction and accumulation of foreign materials in the body. The use of extracorporeal circulation (Chang, T. M. S., "Semipermeable aqueous microcapsules (Artificial cells) with emphasis on experiments in an extracorporeal shunt system., Trans. Am. Soc. Artif. Intern. Organs 12:13-19, (1966)) avoids this problem. However, it is not always easy or convenient to use extracorporeal procecures. This is especially in infants and when repeated treatments are required.
Another approach is the use of immobilized enzymes for oral administrations. Microencapsulated enzymes has been used to prevent destruction from tryptic enzymes in the intestine (Chang, T. M. S., "Artificial Cells", Charles C. Thomas Publisher, Springfield, Ill., U.S.A., (1972)). However, orally administrated enzymes have only very limited applications:
(1) use of enzyme to remove unwanted substrates from food; for example, the use of lactase to remove lactose, or phenylalanine ammonia lyase to remove phenylalanine (Bourget, and Chang, T. M. S. "Phenylalanine ammonia-lyase immobilized in semipermeable microcapsules for enzyme replacement in phenylketonuria, FEBS Letters 180:5-8, 1985); PA0 (2) use of tryptic enzymes to supplement patients with deficiency in pancreatic secretions to allow for digestion of food; PA0 (3) use of enzyme to remove substrates equilibrating from capillaries into the GI tract. Urea is the only one thought to equilibrate with sufficient rate across the capillaries into the intestine. Others like uric acid and creatinine are found to diffuse too slowly to be effectively removed. PA0 mucus PA0 mucins- glycoprotein (rich in threonine, serine, proline, also cystine) PA0 pepsin PA0 pepsinogen PA0 gastrin PA0 trypsin PA0 chymotrypsin PA0 carboxypeptidase PA0 deoxyribonuclease PA0 ribonuclease PA0 elastase PA0 lipase PA0 amylase PA0 hydrolase PA0 phospholipase PA0 vasoactive intestinal polypeptides PA0 mucus PA0 enterokinase PA0 secretion PA0 cholecystokinin PA0 aminopeptidases PA0 dipeptidases PA0 dipeptidyl aminopeptidase PA0 dipeptides PA0 tripeptides PA0 tetrapeptides PA0 oligopeptides
In the small intestine, proteins and peptides from food are broken down into amino acids. Amino acids are then absorbed from the intestine into the portal circulation into the body. Little interest was paid to the movement of amino acids from the body into the intestine. Most authors assume that these movements are by passive diffusion or other membrane transport mechanisms. These proposals are mainly based on studies using isolated intestinal segments or brush border membrane vesicles (Christensen, H. N., Feldman, B. H. and Hastings, A. B., "Contractive and reversible character of intestinal amino acid transport", Am. J. Physiol 205(2) 255-260 (1963); Bertellot, A., Malo, C. "Maladies membranaires de l'intestin et du rein", Medicine/sciences I:427-434 (1985); Bertellot, A., Khan, A. H., Ramaswamy, K., "K and Na gradient dependent transport of L-phenylalanine by mouse intestinal brush border membrane vesicles" Biochmi. Biophys. Acta 691:321-331 (1982); Cooke, H. J., Fankuche, L., Cooke, A. R. "Tryptophan transport by isolated newborn rabbit jejunum" The American Physiology Society 239: G306-G310 (1980); Samiy, A. H., Spencer, R. P. "Accumulation of L-phenylalanine by segments of small intestine" Am. J. Physiol. 200(3): 505-507 (1961)).
It is known that proteins and peptides are secreted into the intestinal tract. However, no studies were carried out on the extent of contribution of this to the amino acid sources.